1. Field of the Invention
This invention relates to a lead frame and, particularly, to a multi-layer lead frame having at least one metal plate or plane which is used for a semiconductor device.
2. Description of the Related Art
A conventionally known multi-layer lead frame for a semiconductor device comprises at least one metal plane connected via an insulation layer to the bottom surface of inner leads. The use of such a multi-layer lead frame enables the mounting of a relatively high-power semiconductor chip on the metal plane, due to heat dispersion from the metal plane. Also, the metal plane can be used as a ground layer or a power-supply layer to improve the electrical characteristics of the semiconductor device.
FIG. 6 shows a conventionally known multi-layer lead frame which includes three layers, i.e., a signal layer 5, a power supply layer 6 and a ground layer 7, which are affixed or laminated to each other, in turn, by means of electrically insulation films 8 and 8, respectively, made of, for example, polyimide.
In the illustrated example, the lowermost layer formed as the ground layer 7 and the intermediate layer formed as the power supply layer 6 can be connected to a ground voltage and a power voltage, respectively, by means of bonding-wires extended from the optional positions of a semiconductor chip to the ground layer 7 and the power supply layer 6, respectively, since these layers are formed as planes.
In order to connect the power supply layer 6 and the ground layer 7 to the power voltage and the ground voltage, respectively, the power supply layer 6 and the ground layer 7 are provided with respective projections or electrodes 9 and 10 protruded from the outer edges thereof for electrically connecting them to the power supply leads and the ground leads, respectively, of the signal layer 5 by such as a resistance welding or the like. In practice, a plurality of such projections or electrodes 9 and 10 are formed substantially equidistantly on the power supply layer 6 and the ground layer 7, respectively. These projections 9 and 10 are preformed on the power supply leads and the ground leads, respectively, so as to accurately align with a plurality of power supply and ground leads, respectively, of the signal layer 5.
However, a process of resistance welding for connecting the electrodes 9 and 10 of the power supply layer 6 and the ground layer 7 to the power supply leads and the ground leads of the signal layer 5 is quite different from a process for adhering the signal layer 5, the power supply layer 6 and the ground layer 7 together so as to form a laminated layer and, therefore, such a process needs a special welding apparatus for the purpose. Thus, more laborious works is required. In addition, when resistance welding is conducted, it is difficult to obtain a fully sufficient uniformity in stability and strength of welding. Therefore, it is sometimes difficult to obtain a necessary strength of welding or a good appearance.